In the road finishing machine disclosed in EP 2 325 390 A, hydraulic proportional valve technology is employed for moving each extendable screed part to be able to vary the moving speed as required and optionally load-independently if in the laying operation a neat peripheral edge adjustment in the surfacing is required or an obstacle must be driven around without any noteworthy change of the laying travel speed of the road finishing machine. Using proportional valve technology, the speed of the movement actuator can in principle be continuously varied because the proportional magnet adjusts the amount of hydraulic oil that determines speed in response to the value of the control current. The control of the proportional magnet is suitably placed in one external control stand each at the screed, e.g. at the extendable screed part, so that it can be correspondingly handled by an operator who is standing on a platform at the screed and driving along or who is walking next to the screed with visual inspection of the external end of the extendable screed part. While proportional valve technology for the movement actuators of the extendable screed parts promises a considerable improvement in the surfacing quality, unexpected problems occur in practice resulting, among other things, from operational vibrations of the control and/or the relative movements between the accompanying operator and the external control stand and aggravating a sensible and precise handling of the control and thus the speed control of the extendable screed part.
U.S. Pat. No. 5,362,176 A, discloses a road finishing machine in which the movement actuators of the extendable screed parts are not hydraulically controlled by proportional valve technology but by black-and-white valve technology, i.e. where only On/Off-speed control at a constant moving speed is possible. For example, an automatic system is provided for a neat edge adjustment of the laid surfacing. The system operates with ground scanning sensors and a timing/delay control section with two control relays. Depending on the signals of the ground scanning sensors, a control relay is excited which automatically adjusts the control current for the black-and-white magnet of the respective movement actuator. Since the movement actuator can only drive at one speed, the laying travel speed of the road finishing machine must be correspondingly varied for a neat adjustment or when driving around obstacles, optionally involving losses in the surfacing quality.